As NASA's space shuttle program draws closer to its end, the production plant responsible for building the space plane fleet's giant external fuel tanks is shutting down.

Lockheed Martin had been building the huge, rust-colored tanks for NASA since 1973 at the space agency's Michoud Assembly Facility in New Orleans. The company announced Thursday (Sept. 30) that tank production at Michoud has stopped, with a total of 136 of the behemoths built over the years.

The final shuttle fuel tank arrived at NASA's Kennedy Space Center in Florida on Sept. 27. It will help fuel the last launch of Endeavour, currently scheduled for February, 2011.

NASA currently has two shuttle missions on the calendar: the shuttle Discovery's Nov. 1 launch to the International Space Station and Endeavour's mission, which was slated to be the last hurrah for the orbiter fleet. [Photos: Discovery's last launch pad trip.]

However, Congress on Wednesday approvedplans for an extra shuttle flight to be flown by the Atlantis orbiter sometime next year as part of its NASA authorization bill. The bill still has to be approved by President Obama and pass through the appropriations process, NASA officials said.

The fuel tank for that mission was delivered to Kennedy Space Center in July, NASA officials said.

Reduction in workforce
The tank production shutdown has resulted in substantial layoffs at Lockheed. On Jan. 1, 2010, the company employed more than 1,400 workers at Michoud. That number is now down to about 600, according to Lockheed spokesman Marion LaNasa.

"We had 300 people that were released from the program today," he told SPACE.com Thursday.

Of the remaining 600, about 200 will work on the company's Orion project, which is building a manned space capsule envisioned as a replacement for the space shuttle. Lockheed is carrying out some Orion manufacturing and testing work at Michoud.

About 150 Lockheed employees at Michoud will continue to support shuttle launch and landing activities, LaNasa said.

"They'll do anything and everything that's needed to disposition a tank for flight," he said.

Such work could include repairing a tank that becomes damaged, as happened in February 2007, when a hailstorm pelted the shuttle Atlantis' tank as it sat on the launch pad.

The rest of the Michoud employees will perform various other jobs, LaNasa said.

Building a behemoth
An external fuel tank stands 154 feet (47 meters) tall, measures about 28 feet (8.5 meters) across and weighs almost 59,000 pounds (26,818 kg) when empty. Tanks are used to hold the 535,000 gallons of super-chilled liquid oxygen and liquid hydrogen propellant consumed by a shuttle during the 8 1/2-minute ascent into orbit.

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Over the years, Lockheed has slimmed these behemoths down significantly, allowing shuttles to carry more payload.

The first tanks the company produced weighed about 76,000 pounds (34,545 kg). The first two shuttle tanks were painted white, though it was later deemed unnecessary. By 1982, the weight had been reduced to 66,000 pounds (30,000 kg). The current super-lightweight version debuted in 1998.

Over the last seven years, NASA and Lockheed Martin have worked continually to increase safety and reduce the amount of insulating foam covering the shuttle fuel tanks. Foam debris shed during launch damaged the space shuttle Columbia's heat shield during its final flight in 2003, leading to the loss of the shuttle and its seven-astronaut crew.

Since then, the tanks were modified to reduce the amount of foam insulation shed during launch. The tanks are meticulously inspected before launch and carry cameras to show any foam loss events during liftoff. Astronauts also inspect their shuttle heat shields after reaching orbit to seek out any new dings or damage. NASA resumed shuttle flights after the 2003 Columbia accident in July 2005.

Disposable fuel tanks
When space shuttle external tanks are filled with liquid hydrogen and liquid oxygen propellant, they weigh nearly 1.7 million pounds (772,727 kg), Lockheed officials said. At the launch pad, they are flanked by twin solid rocket boosters strapped to the tank's sides. The actual spacecraft, called the orbiter, attaches to mounts on the tank.

During launch, tank and boosters are jettisoned and fall back to Earth after a shuttle's initial push to the sky. Unlike the boosters, however, the external tank is not collected and reused. Instead, the tanks are discarded to burn up in Earth's atmosphere.

The next tank to fly will be used by the space shuttle Discovery, which is scheduled to lift off next month on an 11-day mission to the space station. Discovery will deliver a storage room to the space station, along with a humanoid robot assistant for the outpost's astronaut crew.

Discovery is currently attached to its external tank and rocket boosters and standing atop its seaside launch pad awaiting its Nov. 1 launch date.

Endeavour is slated to deliver a nearly $2 billion astrophysics experiment designed to study cosmic rays from the space station. The mission will also deposit some spare parts for the outpost.

In all, NASA will have flown up to 135 shuttle missions since the first orbiter launch in 1981.